Oct. 15, 2009 -- Marijuana smokers who take up the habit in their teens, as
well as those who smoke daily or nearly every day, are at the greatest risk for
dependence and other ill health effects, according to a new review of marijuana
use worldwide by Australian researchers.

About 9% of those who ever use pot will become dependent, the researchers
estimate.

But a U.S. expert on the health effects of marijuana says the public health
impact of pot is "miniscule" compared to the effects of alcohol. Likewise,
the Australian experts acknowledge that the public health burden of marijuana
is probably modest compared to that of alcohol, tobacco, and other illicit drugs.

About one in 25 people ages 15 to 64 have used marijuana worldwide,
according to Wayne Hall, PhD, professor of public health policy in the School
of Population Health, University of Queensland, Brisbane, Australia, and the
co-author of the review published in The Lancet.

"It is an update review [and] was motivated by the collection of better
epidemiological information over the past decade on the health risk of
cannabis," Hall says in an email interview with WebMD.

Hall searched the medical literature for the past 10 years, looking for
studies that focused on adverse health effects of marijuana.

In the U.S., marijuana use peaked in young adults in 1979, then decreased
until the early 1990s, when it again increased before leveling off toward the
end of the 1990s, Hall says.

But marijuana use varies globally, Hall found. "Trends in use over the past
decade have varied between regions and countries," Hall says. Use has
stabilized or fallen in many developed countries while increasing in some
developing countries, he says.

"The main points of the paper are that evidence has strengthened for the
existence of a dependence syndrome, an increased risk of motor vehicle
accidents if users drive while intoxicated, impaired respiratory function in
daily smokers, psychoses in young people who begin in their mid-teens and use
daily or near daily, and poorer psychosocial outcomes in adolescents who
initiate early and become regular users," Hall tells WebMD.

Marijuana and Health

In the medical literature review, Hall found:

About 9% of those who ever use marijuana become dependent. But the risk
rises to one in six if use begins in teen years. About 10% of ever-users become
daily users, and 20% or 30% become weekly users.

Driving after smoking marijuana increases the risk of a motor vehicle
accident by two to three times, research suggests, while driving after drinking
alcohol increases accident risk six to 15 times.

Symptoms of chronic bronchitis were more commonly reported among regular
marijuana smokers than nonsmokers. The smokers reported wheezing, chronic
cough, and production of sputum.

Studies looking at a possible link between an increased risk of cancers in
the upper respiratory tract and marijuana use have produced mixed findings,
with some finding a link and others not.

Marijuana use increases the heart rate, and adults with existing heart
disease may be at higher risk of a heart attack after pot use, some research
suggests.

Regular marijuana users, especially if they started at a young age, are
more likely to later use other drugs such as heroin and cocaine, according to
some studies.

Regular and heavy pot use has been linked with problems in memory,
attention, and verbal learning, but researchers aren't certain whether those
changes are transient and disappear once marijuana use is stopped.

The use of marijuana by age 18 is linked with more than a doubling of risk
for a later diagnosis of schizophrenia, according to a large Swedish
study.

Early marijuana use, before age 15, has been linked with school dropouts.
But researchers are unsure exactly how to explain the association. One
possibility: poor school performance can trigger the pot use, which in turn
makes school performance even worse.

Smoking pot during pregnancy has been linked with underweight babies, but
there is little evidence it's linked to birth defects.

Avoiding Health Risks of Marijuana

Hall notes that some people are at
greater risk for adverse health effects from smoking marijuana, especially
"young persons under the age of 18 and ... persons with any disease or
condition (for example, pregnancy, cardiovascular or respiratory disease,
mental illness, or other types of substance abuse) which increases their
vulnerability to its adverse effects."

Information from the new review doesn't reflect much of a change, says Mitch
Earleywine, PhD, an associate professor of psychology at the University at
Albany State University of New York. He's the author of Understanding
Marijuana: A New Look at the Scientific Evidence.

"The rates [in the new report] are literally identical to data that came
out in the 1990s," says Earleywine, who reviewed the Australian report for
WebMD.

He cautions that the new review shouldn't be blown out of proportion. "The
impact [of marijuana] is miniscule compared to alcohol," he says. "'Nobody
turns to prostitution because of a cannabis habit."

The research findings about heavy early use and its effects are troubling,
Earleywine says. "That's a genuine concern." Teens who begin smoking at 13 or
15, he says, "are really putting themselves at risk for a lot of problems.
Those seem to be the kids more likely to drop out of high school."

SOURCES: Wayne Hall, PhD, professor of public health policy, School of Population
Health, University of Queensland, Brisbane, Australia.

Hall, W. The Lancet, Oct. 17, 2009: vol 374: pp1383-1391.

Mitch Earleywine, PhD, associate professor of psychology, University at Albany, State University of New York; author, Understanding Marijuana: A New Look at the Scientific Evidence.